When two surfaces are brought in contact with each other, a transfer of electrons may occur resulting in a residual static electrical charge when the surfaces are separated. This phenomena is known as triboelectricity. If the surface is composed of a material that is a conductor, the electrons will dissipate quickly thereby eliminating the excess charge. On the other hand, if the surface is composed of a material that is an insulator (a dielectric), the surface charge takes much longer to dissipate.
Thermoplastic polymers, however, are typically excellent insulators, having an extremely high surface resistivity of more than 10.sup.14 ohms/square, and thus they are unsatisfactory for uses that require an antistatic nature. As the polymers are nonconductive, they accumulate high charges promoting an attraction for dust and dirt, and they can discharge to any lower potential body with which they come in contact. To modify a polymer so that it will have antistatic characteristics, the resistivity of it must be decreased, i.e. the conductivity is increased which in turn causes an increase in the rate of static dissipation. Increase in conductivity has been accomplished in the past by the use of antistatic agents to promote static-charge decay of surfaces thereby reducing clinging effect, eliminating spark discharge, and preventing accumulation of dust.
It is well known that static charge can be reduced by increasing the moisture content of the atmosphere, and thus the approach in the past has been to use an antistatic agent which will chemically modify the polymer to impart hydrophillic properties to it by providing functional groups that attract moisture to it. For instance, it is well known to apply external antistatic agents onto polymers by conventional coating methods. Also, it is well known to apply internal antistatic agents which are volume dispersed in the polymer; i.e. incorporated into the polymer by compounding or extrusion prior to or during molding or film-forming operations. These agents work by migrating to the polymer surface. This migration is colloquially referred to in the art of polymer chemistry as a "blooming" effect. When the antistatic agent has not remained volume dispersed but instead has bloomed to the surface, the mechanism for moisture attraction is the same as with the external antistatic agents. The atmospheric moisture is attracted causing decay of static charges. Accordingly a high rate of blooming is required.
The following known antistatic agents are believed to function in the above-mentioned manner.
An example of an external antistatic agent is described in U.S. Pat. No. 3,223,545 to Gallaugher et al which discloses a dialkanol amide of the formula ##STR1## wherein R is a C.sub.6 to C.sub.16 alkyl and n is an integer from 2 to 4, dispersed in a volatile liquid which is applied to the surface of a solid polymer.
Another external antistatic agent is described in U.S. Pat. No. 4,268,583 (1981) to Hendy which relates to an antistatic film having a polypropylene substrate and a polymeric heat-sealable surface layer on which is present an antistatic composition comprising (a) a quaternary ammonium compound, such as choline chloride, (b) an organic polyol containing at least two free hydroxyl groups, such as glycerol, (c) a glyceride of a long chain fatty acid, such as glyceryl monostearate, and, optionally, (d) an ethoxylated amine salt, such as an ethoxylated tallow amine sulphate. The composition is conveniently applied directly to the surface of a polymeric extrudate which is subsequently drawn to yield an oriented film, but at least the glyceride may be preblended into the film-forming polymer.
One example of an internal antistatic agent is described in U.S. Pat. No. 3,220,985 to Breslow which discloses modifying hydrocarbon polymers with a monosulfonazide of the formula RSO.sub.2 N.sub.3, where R is an organic radical inert to the modification reaction. For instance, to an acetone slurry of finely divided polypropylene is added para-toluene sulfonazide, followed by agitation at room temperature to evaporate the acetone solvent. The resultant is then heated at 160.degree. C. for 2 hours.
Another internal antistatic agent is described in U.S. Pat. No. 3,164,481 to Shibe which discloses combining a quaternary ammonium benzosulfimide with a plastic. (For clarity, it is mentioned benzosulfimide is also known as saccharin.) For instance, in Shibe is disclosed Epolene E (a polyethylene supplied by Eastman Chemical Products, Inc., Kingsport, Tenn.) melted together with dodecyl benzyl trimethyl ammonium benzosulfimide and the molten resultant is spread out in a sheet.
Also of interest is the internal antistatic agent described in U.S. Pat. No. 3,576,649 to Brazier. This patent relates to a package for electrically non-conductive pulverulent material. The package has an inner layer of heat sealable ethylene polymer to which is added a fatty acid amide in a quantity such that the pulverulent material being packaged will not be attracted to film areas that are to be heat sealed to form a completed package, such as may occur if a charge of static electricity is generated when film from which the package is made passes through a packaging machine of the form-and-fill type.
Also of interest is the internal antistatic agent described in U.S. Pat. No. 3,441,552 to Rombusch et al. The patent discloses incorporating an alkoxypropylamine of the formula ##STR2## into a polyolefin where R.sub.1 represents an alkyl, alkenyl, alkylcycloalkyl, aryl, alkylaryl, or alkenylaryl group of 6 to 25, preferably 8 to 18 carbon atoms in the alkyl or alkenyl moieties and 4 to 18, preferably 6 to 12 carbon atoms in the cycloalkyl moiety, and 6 to 14, preferably 6 to 10 carbon atoms in the aryl moiety; R.sub.2 and R.sub.3 can reach represent a hydrogen atom, or an alkyl or alkenyl group of 1 to 5 carbon atoms. For instance, 100 g of octadecyloxy-propyl-N,N-dimethylamine are homogenized in a blender with 10 kg of polypropylene. The resultant is granulated and injection mold plates are produced from the granulation.
Another internal antistatic agent is disclosed in U.S. Pat. No. 4,554,210 (1985) to Long et al, which claims a laminated, anti-static, heat sealable packaging material for packaging and preventing contamination of an electrostatically sensitive item packed in said packaging material, comprising: a first outer layer of heat-sealable, semi-conductive polyethylene having a surface resistivity at least 1.times.10.sup..noteq. ohms per square; a second outer layer of heat-sealable, semi-conductive polyethylene having a surface resistivity at least 1.times.10.sup.16 ohms per square; and a middle layer of heat-sealable polyethylene impregnated with a sloughable, electrically-conductive material providing said middle layer with a volume resistivity no more than 1.times.10.sup.3 ohms per centimeter, said middle layer bondedly sandwiched between said first and second layers to prevent said electrically-conductive material sloughing from said middle layer to contaminate an electrostatically sensitive item when said item is heat-sealably packaged in said packaging material.
Another internal antistatic agent is disclosed in U.S. Pat. No. 4,600,743 (1986) to Shizuki et al which describes an antistatic fiber obtained by melt spinning of a fiber-forming thermoplastic polymer containing at least one of polyoxyalkylene glycol and its derivatives in an amount of not less than 0.5% by weight, characterized in that said fiber has a half life time of electric charge leakage of not more than 150 seconds before and after treatment with a weight decreasing agent and, when treated with a weight decreasing agent, provides a number of streaks arranged in parallel in the lengthwise direction at the surface.
Another internal antistatic agent is disclosed in U.S. Pat. No. 4,117,193 (1978) to Tsuchiya et al, wherein there is disclosed a novel composite film prepared by melt extrusion laminating a polymer blend composition comprising a low-crystalline resin of an ethylene-butene copolymer and a polyolefin resin having incorporated therein a lubricant and an anti-blocking agent onto a surface or surfaces of a uniaxially stretched polypropylene film followed by stretching the laminate film in the direction perpendicular to the direction in which said polypropylene has been stretched and optionally subjecting the resulting film to corona discharge treatment.
The antistatic agents useful in the present invention are substantially non-hygroscopic and substantially-non-migratable. Thus, unlike with the previous antistatic polymeric compositions, the antistatic properties of the antistatic polymeric composition of the present invention are substantially independent of ambient relative humidity and the agent substantially remains volume dispersed in the polymer. Accordingly, the problem of surface contamination and corrosion when a static sensitive device was in contact with the previous antistatic polymeric compositions is obviated. For instance, the present polymer containing the agent may be made into a single and/or multi-ply film. Such films have an increased tendency to dissipate electrostatic charges. For instance, the composition may be admixed in and/or extruded together with polymers known for their strength such as the polyethylenes, for instance ethylene vinyl acetate (EVA) or linear low density polyethylene (LLDPE), to make a film. Such a film is useful for making packaging, such as an over-wrap or bag, for electronic devices that are sensitive to static electricity. Such a film may also be fashioned to have cushioning characteristics by using bubble cap (also known as air cushioning) machinery such as that described in U.S. Pat. No. 4,576,669 and U.S.Pat. No. 4,579,516, both to Caputo, to make a bubble cap packaging which provides cushioning and then bagging or wrapping a circuit board therewith. Such a cushioning bubble cap material is also useful in lining a portable work station used for storage and transportation of static electricity sensitive devices such as the work stations disclosed in U.S. Pat. No. 4,494,651 issued in 1985 to Malcolm. Also such a film, which may or may not be in a cushion form, is useful to make a package for devices in a medical operating room where explosive oxygen and/or ether are present and thus protection from static electricity must be provided.
Therefore, it is an object of the present invention to provide an antistatic polymeric composition wherein the antistatic agent is substantially non-migratable and substantially remains volume dispersed in the polymer. It is a further object to provide the antistatic polymeric composition in single-ply or multi-ply film useful for wrapping static sensitive devices. It is also an object to provide such films having an increased tendency to dissipate electrostatic charges.